linux-kernel - RE: [PATCH 00/11] pragma once: treewide conversion

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Message-ID: <4835ec1d2ecc40b285596288a0df4f47@AcuMS.aculab.com>
Date:   Thu, 4 Mar 2021 13:55:12 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Tom Tromey <tom@...mey.com>
CC:     Alexey Dobriyan <adobriyan@...il.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sparse Mailing-list <linux-sparse@...r.kernel.org>
Subject: RE: [PATCH 00/11] pragma once: treewide conversion

>  (a) the traditional include guard optimization HAS NO HIDDEN SEMANTIC
> MEANING. It's a pure optimization that doesn't actually change
> anything else. If you don't do the optimization, absolutely nothing
> changes.

And if the parser is well written the optimisation is probably
irrelevant compared to the compile time.

OTOH that probably requires using mmap(), memchr('\n') to look
for line starts, a fast search for '[ ]*#' followed by else/endif
and a final horrid backwards check for a continuation line.

That optimisation will generally speed up header file processing.

	David

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